Overview
A nitro compound is an organic molecule that carries the nitro functional group, written as -NO2. The nitro group links a nitrogen atom to two oxygen atoms and to the remainder of the molecule. Nitro compounds occur widely in synthetic chemistry and appear in materials as diverse as dyes, pharmaceuticals and explosives. For a basic entry about nitro-containing molecules see nitro compound reference.
Structure and bonding
The electronic structure of the nitro group is best described by resonance between several canonical structures rather than by a single Lewis formula. One common depiction shows nitrogen bearing a formal positive charge and one oxygen carrying a negative charge. In more accurate valence-bond and molecular-orbital descriptions the three atoms are approximately sp2-hybridised and the two N–O bonds share electron density through delocalization. For introductions to the group and its atoms see nitro group, nitrogen and oxygen. The bond linking nitrogen to the rest of the molecule is often indicated as N–R; that connection is central to reactivity (nitrogen bonding and resonance) and to how the group withdraws electron density from the rest of the molecule (bonding details).
Preparation and typical reactions
Aromatic nitro compounds are commonly made by nitration: electrophilic substitution using nitric acid, often activated by sulfuric acid, to install -NO2 on an aromatic ring. Aliphatic nitro compounds can arise from nucleophilic substitution, oxidation of amines, or addition to alkenes. Nitro groups are strongly electron-withdrawing and affect acidity and reactivity: nitroalkanes have acidic α-hydrogens and undergo condensations, while nitroaromatics are susceptible to nucleophilic aromatic substitution when activated. Reduction of nitro groups yields amines, a principal route to many anilines and other amine derivatives.
Applications and importance
- Intermediates in synthesis: precursors to dyes, pharmaceuticals and agrochemicals.
- Explosives and propellants: several energetic molecules (e.g., TNT, nitroglycerin) contain nitro functionalities that release energy on rapid decomposition.
- Specialty materials: nitro compounds contribute to electronic and chromophore properties in polymers and dyes.
Safety, distinctions and notable facts
Nitro compounds vary widely in hazard. Some are relatively stable synthetic intermediates; others are shock-sensitive, toxic or oxidizing. The nitro group should be distinguished from related functionalities such as nitroso (–NO), nitrate esters (R–O–NO2) and azo groups. Because the nitro group withdraws electron density, its presence dramatically alters physical and chemical behavior compared with the parent hydrocarbon, influencing acidity, reactivity and environmental fate.
Further reading and technical data can be found in specialized organic chemistry sources and safety datasheets: see general summaries at nitro compound reference and deeper discussions of bonding and resonance at nitro group and bonding details. For atomic descriptions consult nitrogen and oxygen, and for discussions of nitrogen connectivity see nitrogen bonding.